Laser processing systems employed for processing dynamic random access memory (DRAM) and other devices commonly use a Q-switched diode pumped solid state laser. When processing memory devices, for example, a single laser pulse is commonly employed to sever an electrically conductive link structure. In another industrial application, Q-switched diode pumped solid state lasers are used to trim resistance values of discrete and embedded components.
Some laser processing systems use different operating modes to perform different functions. For example, the ESI Model 9830 available from Electro Scientific Industries, Inc. of Portland, Oreg., the assignee of the present patent application, uses a diode pumped Q-switched neodymium-doped yttrium vandate (Nd:YVO4) laser operating at a pulse repetition frequency of approximately 50 kHz for laser processing of semiconductor memory and related devices. This laser system provides a pulsed laser output for processing link structures and a continuous wave (CW) laser output for scanning beam-to-work targets. As another example, the ESI Model 9835, also available from Electro Scientific Industries, Inc., uses a diode pumped Q-switched, frequency-tripled Nd:YVO4 laser for laser processing semiconductor memory and related devices. This laser system uses a first pulsed laser output at a PRF of approximately 50 kHz for processing link structures and a second pulsed laser output at a PRF of approximately 90 kHz for scanning beam-to-work targets. In some systems, higher PRFs (e.g., approximately 100 kHz) are also possible. Generally, the pulse widths of laser pulses generated by such laser systems are functionally dependent on the PRF selected and are not independently adjustable based on differences between target structures or other process variables.
Some systems have used tailored pulse shapes to process workpieces. For example, U.S. Pat. No. 7,348,516, which is assigned to the assignee of the present patent application, describes one such laser technology in which laser processing of conductive links on memory chips or other integrated circuit (IC) chips is accomplished by laser systems and methods employing laser pulses with a specially tailored intensity profile (pulse shape) for better processing quality and yield. As another example, U.S. Pat. No. 7,126,746, which is assigned to the assignee of the present patent application, describes a method of employing a laser processing system that is capable of using multiple laser pulse temporal profiles to process semiconductor workpiece structures on one or more semiconductor wafers. Generally, in a link processing system there are several laser pulse parameters that define the laser-material interaction. In addition to laser wavelength, these parameters include both spatial characteristics (e.g., spot size, waist location, and ellipticity) as well as temporal characteristics (e.g., peak power, pulse energy, pulse width, and pulse shape). In order to provide a robust process that can be repeated on multiple link processing systems, the laser pulse parameters may be: (a) passively controlled by design and measured during manufacturing to verify performance; (b) controlled through calibrations performed periodically; or (c) actively measured and controlled with a feedback loop. In certain laser processing systems, such as a tailored pulse laser processing system, method (c) may provide more flexibility than methods (a) or (b).
Typical laser processing systems generally monitor the various laser parameters in different ways. For example, Table 1 summarizes the current state of the art with respect to laser pulse process parameter control.
TABLE 1TypeParameterMethodNotesSpatialSpot Size(b) CalibrationCalibrated as part of the “ProgrammableSpot (PS) Calibration” duringmanufacturing and preventativemaintenance. Systems can be configuredto monitor spot size periodically, butfeedback correction is generally notallowed.SpatialWaist Location(c) FeedbackMeasured during wafer alignment andcontrolled at runtime by moving theobjective lens with z-stage.SpatialEllipticity(a) PassiveMeasured during PS Calibration. Theremay also be automated adjustments.SpatialAsymmetry(a) PassiveMeasured during PS Calibration. Theremay also be automated adjustments.TemporalPulse Energy(c) FeedbackDefault system configuration allows forruntime feedback of pulse energy asmeasured by a pulse detector and ascontrolled with an acousto-optic modulator(AOM).TemporalPeak(c) FeedbackSystem can be configured to allow runtimePower(optional)feedback of peak power instead of pulseenergy using the pulse detector. This isgenerally optional.TemporalPulse Width(a) PassiveConsidered constant for a given laserarchitecture operating at the same laserrepetition rate. Measured duringmanufacturing to confirm that theparameter is within specification.TemporalPulseN/AFor solid state lasers, peak height (e.g.,Shapepeak power), pulse energy, and pulsewidth sufficiently describe the temporalshape. For a tailored pulse system,however, this is not true.
FIGS. 1A and 1B are example temporal pulse shapes of laser pulses generated by typical solid state lasers. The pulse shown in FIG. 1A may have been shaped by optical elements as is known in the art to produce a square-wave pulse. As shown in Table 1 and in FIGS. 1A and 1B, a typical solid state pulse shape is well described by its peak power, pulse energy (time integration of the power curve), and pulse width measured at a full-width half-maximum (FWHM) value. Feedback from a pulse detector may be used to determine pulse energy and/or peak power. The pulse detector used for feedback may include a diode coupled to an analog peak capture-and-hold circuit for peak power sensing. The pulse detector may also include an analog integration circuit for pulse energy measurements. Unlike using a solid state laser to generate typical laser pulses, tailored pulse technology using, for example, a fiber laser or master oscillator fiber power amplifier (MOFPA) allows for pulse shapes that are not adequately described by typical peak power, pulse energy, and pulse width metrics. For example, FIGS. 2A and 2B are example temporal pulse shapes of tailored laser pulses generated by a dynamic laser pulse shaper and power amplifier according to one embodiment. As shown in FIG. 2A, a peak power P1 of a leading spike on the power curve does not describe the height of a plateau or “seat” power P2 on the so-called chair-shaped pulse. Further, some tailored pulses may have multiple spikes or multiple plateaus that are not described by the peak power P1. For example, as shown in FIG. 2B, a peak power P1 of a spike does not describe the height of a first plateau power P2 or a second plateau power P3. In addition, as discussed below, a pulse width based on a FWHM metric may provide the same result for a plurality of chair-shaped pulses with different “seat” lengths.